Hoppy

A bit off topic, now. Sorry peeps.

Mik, yeah I realised it was you when, in a moment of inspiration, I thought I'd check your profile. I too read that American piece about a water-wheel as John F forced me to The guy obvioulsy knew what he was on about, but by the time I'd got to the end I'd forgotten what the question was.

Mik, seriously, some motoring mags could use your knowledge and writings skills. Most mags are technically very superficial. I was reading a Greek mag the other day, well, looking at the pictures anyway (don't ask why - long story). There was an article explaining roll-centres and stuff, and a pull-out chart so you could measure you car's bump-steer! Can't see that ever happening in AutoExpress.

John, thanks for the insight into why diesels are heavy. I forgot they run 50:1 compression ratios or something Are you involved in updating the SIDC faq? I think some basic technical features would go very well in there. You're quite handy with a pen as well... just a shame you know bu99er all, mate

Best regards,

Richard.

[Edited by Hoppy - 4/1/2002 4:58:01 PM]

LG John

Ok, here is a question for the technical minds! Clearly torque and bhp are both just measurements of what an engine is doing.

For me I find torque easy to identify when driving a car - jump in a 330d with a shed load of torque at around 2500rpm and you feel it as a big thud in your back as the car drags itself and anything in it toward the horizon.

So where does that leave bhp, how do you 'feel' bhp when your driving? Can the two even be seperated like that? Anybody got a clue?

ukscooby

BHP is a measurement of 'power' being developed by the engine at particular revs. There are different measurements of power in different countries. I think the germans use DIN which is the power measured of an engine with all unecessary ancillaries removed. Torque is a measurement of 'turning force'. You create torque when you use a spanner, so on a car its a measurement of turning force acting on the crankshaft.

I may be corrected by a few peeps but i hope that helps!

Al

Shadowfax

Whilst torque is a measurement of "turning force", Horsepower (usually measured on a brake, hence brakehorsepower) is torque times the rate at which it is produced. In Imperial measure, torque is measured in lbs.ft,, and 1 x BHP is 33,000lbs.ft./min.

In practical terms, both have the same effect when driving.
Assuming overall gear ratios are the same, 100lbs.ft. of torque @ 5000 rpm will create exactly the same power as 200lbs.ft. of torque @ 2500 rpm.

The german DIN standard of measuring horsepower is I think almost universally used these days, and aims to provide a "genuine" figure at the flywheel.

This may be amended by later postings, but I think I am broadly correct.

NBW

Torque http://www.howstuffworks.com/fpte3.htm

Power http://www.howstuffworks.com/fpte5.htm

chrisp

Torque moves cars but BHP sells 'em

NBW

LOL @ chrisp.... too true Sir....

john banks

A diesel with a lower rev limit and much longer gears may develop 200-300 lbft of torque but it won't move down the road like a petrol engine with the same torque and a 50% higher rev limit. So 150 lbft at 6000 RPM will be far more (50% more) effective (in terms of power at wheels) than 300 lbft at 2000 RPM if you stay in the power band. The torque just drops away in all diesels when the fun is starting - even a 330d.

Perhaps area under torque or power curve is a more useful measurement of a car's performance than one single peak measurement of power or torque?

Power does correlate best with 1/4 mile times, terminal speeds, 0-100, 60-100, 30-70 through the gears etc. Torque will correlate best with 50-70 in 5th for example.

A car with lots of torque low down will be easy to drive and you can be much lazier with gear changes.

My wife (like your average driver) would be faster in a typical 150 BHP turbo diesel than a 250 BHP petrol car as she never seems to go over 3000 RPM.

Hoppy

Simple question. Uncomplicated answer.

Torque is a measure of the engine's twisting force (say per revolution). BHP is a measure of torque x speed/time. So the more revolutions you can turn per minute, the more BHP you've got at the wheels. That's why you change down to overtake, as the gearbox multiplies the torque with greater engine speed, making you accellerate faster.

Check the dyno pages (click top-right of screen) to see hundreds as BHP/torque curves. Understanding them fully is less easily explained, but the car with the highest BHP figure at any given revs (all other things being equal) will accellerate fastest.

Torque generates BHP, but it's BHP that makes you go

Richard.

scoobypoo

why bother asking such a silly question as a subaru is never going to get much of either

tweenierob

i was once told that torque is (for example) how much weight you can lift and bhp is how quickly you can use the torque to lift the weight?????????
thinking about it, it would mean that a car with low torque but high bhp would struggle to get any speed if put under any load but a low ammount of load could be moved quickly, also if a car has high torque but low bhp (diesel for example) would be able to move the load with ease but not very quickly??
Rant over
Rob
p.s. a 1000bhp 1000lb/ft skyline could pull a house down very quickly lol

Hoppy

Diesels would rule the world if they could rev high enough. Diesel fuel has a higher calorific output, gallon for gallon, than petrol, which is where their economy comes from. If a diesel could rev to, say 15,000rpm, they'd be leading F1 races. Problem is, they can't. No way.

Diesel vs petrol is the perfect example of torque vs bhp. Diesels with massive low-down torque are great for low-revving trucks, but the high-rev potential of petrol engines with a short-ratio gearbox to keep them on song produce even more bhp = faster.

Richard.

PS Edited to add that also, diesels are big, heavy lumps which don't lend themsleves to sports applications. And JB, a car with lots of low-down torque will only be easier to drive because it has lots of low-down bhp

Edited again to ask why are diesels so heavy? I can't think of any reason why this should be, so is it just because if you're driving a lorry a few extra kilos don't matter? Are the modern BMW etc diesels heavier than their cc petrol equivalents?

[Edited by Hoppy - 4/1/2002 2:21:03 AM]

[Edited by Hoppy - 4/1/2002 2:33:39 PM]

LG John

Cheers everyone. I think that Hoppy's little maths piece helped me understand it. So correct me if I'm wrong, but if you have a car with a shed load of torque highish up the rev range then it'll produce a lot of power high up due to the torque x engine speed thing! I'm chuffed now that I understand the relationship between the two.

I've always wished that diesels could rev more, Imagine what a 330d could do if it produced its 288lb ft at 6500rpm!!

mik

Try reading this:-

http://www.seatcupra.net/power.shtm

Gives an analogy you should be able to easily relate with.

BladeRnnr

bhp........the STRENGTH

torque.....THE ABILITY TO USE IT........

SAFELY.........

M.

Hoppy

Saxo Boy, glad you've grasped it. Only problem is, you'll now realise what a lot of rubbish some people talk that don't know which is which, or why Be careful when talking to V-TEC owners, unless you want a fight

As an aside, it could be argued that a turbo petrol engine combines the best attributes of both diesel and petrol engines as the turbo can be made to distort the torque curve of a normally aspirated engine. Again, look at those dyno pages and see massive great dollops of low-down diesel-like torque and then high BHP at 7,000rpm. Mmmmm...

BTW, your 288lbs/ft engine would be producing 356bhp at 6,500rpm

Michael, that link is excellent. I've bookmarked it. The SIDC faq needs something like that. Superbly described, and funny - not easy in a technical article. Did you write it, or is the name a coincidence? If you did, superb; if you didn't, take the credit anyway

Best regards,

Richard.

mik

Hoppy ~ yeah it's "mine"....thanks for the comments!

Wrote it 'cos I was fed up with the b0ll0x a lot of folks talk regarding this subject, and did my best to make it as "accessible" as possible.

Catalyst was an article written by an American gent that Mr Felstead linked to yonks & yonks ago.

johnfelstead

Diesel engines are heavy for a simple reason. They need stronger engine blocks to withstand the higher internal presures generated. Hence heavy engines.

If Subaru made a turbo diesel Impreza i would buy one. If it had the same charicteristics of the 150BHP VW TDi engine it would be a rocket ship!!

There was a Turbo Diesel VW Golf in the BRC a couple of years ago and that kicked the crap out of everything on occasion!

mik

Thanks again hoppy!

I actually tried to get Evo mag interested ~ 5 emails in total without so much as a "thanks but not interested"...... Even HarryM ignored me....

I realised at the time it was probably still a little techy for their publication, but rate their guys and thought they might be able to do smething with it (?). Depsite their words of encouragement however....

I've nothing to do with the SIDC, but (assuming you do) would certainly not have an issue with them using it. Can email you original MSword copy if u like?

Phill

Mik,
Now i understand, i must admit i started to read this thread hoping that somebody could shine a light on it all.....

Right on the money not only have you covered all the questions, that no-body else could answer (the effects of gearing and tyre and wheel size)i scan read this over lunch and i seem to have taken it in ....Genius old boy, Genius !

Very nicely done and thankyou !!

Phill

GP

in answer to what a few people are obviously wondering - the reason a diesel is never ever going to rev with enthusiasm is on a compression-only engine (no spark), you can't advance the ignition.

ergo, it can't maintain torque as the engine speeds up.